3 files changed, 22 insertions, 18 deletions
diff --git a/compiler/rustc_hir_typeck/src/cast.rs b/compiler/rustc_hir_typeck/src/cast.rs
index 59c06cbc5b5..8190095971b 100644
--- a/compiler/rustc_hir_typeck/src/cast.rs
+++ b/compiler/rustc_hir_typeck/src/cast.rs
@@ -721,13 +721,11 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         use rustc_middle::ty::cast::IntTy::*;
 
         if self.cast_ty.is_dyn_star() {
-            if fcx.tcx.features().dyn_star() {
-                span_bug!(self.span, "should be handled by `coerce`");
-            } else {
-                // Report "casting is invalid" rather than "non-primitive cast"
-                // if the feature is not enabled.
-                return Err(CastError::IllegalCast);
-            }
+            // This coercion will fail if the feature is not enabled, OR
+            // if the coercion is (currently) illegal (e.g. `dyn* Foo + Send`
+            // to `dyn* Foo`). Report "casting is invalid" rather than
+            // "non-primitive cast".
+            return Err(CastError::IllegalCast);
         }
 
         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index 541e16e42a7..f9e4a592d92 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -737,8 +737,10 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             return Err(TypeError::Mismatch);
         }
 
-        if let ty::Dynamic(a_data, _, _) = a.kind()
-            && let ty::Dynamic(b_data, _, _) = b.kind()
+        // FIXME(dyn_star): We should probably allow things like casting from
+        // `dyn* Foo + Send` to `dyn* Foo`.
+        if let ty::Dynamic(a_data, _, ty::DynStar) = a.kind()
+            && let ty::Dynamic(b_data, _, ty::DynStar) = b.kind()
             && a_data.principal_def_id() == b_data.principal_def_id()
         {
             return self.unify_and(a, b, |_| vec![]);
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index 1430cfae51f..401b41c796d 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -483,15 +483,19 @@ fn report_negative_positive_conflict<'tcx>(
     negative_impl_def_id: DefId,
     positive_impl_def_id: DefId,
 ) -> ErrorGuaranteed {
-    tcx.dcx()
-        .create_err(NegativePositiveConflict {
-            impl_span: tcx.def_span(local_impl_def_id),
-            trait_desc: overlap.trait_ref,
-            self_ty: overlap.self_ty,
-            negative_impl_span: tcx.span_of_impl(negative_impl_def_id),
-            positive_impl_span: tcx.span_of_impl(positive_impl_def_id),
-        })
-        .emit()
+    let mut diag = tcx.dcx().create_err(NegativePositiveConflict {
+        impl_span: tcx.def_span(local_impl_def_id),
+        trait_desc: overlap.trait_ref,
+        self_ty: overlap.self_ty,
+        negative_impl_span: tcx.span_of_impl(negative_impl_def_id),
+        positive_impl_span: tcx.span_of_impl(positive_impl_def_id),
+    });
+
+    for cause in &overlap.intercrate_ambiguity_causes {
+        cause.add_intercrate_ambiguity_hint(&mut diag);
+    }
+
+    diag.emit()
 }
 
 fn report_conflicting_impls<'tcx>(